A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the bones of the middle ear 103 (malleus, incus, and stapes) that vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. It includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The cochlea 104 forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses which are transmitted to the cochlear nerve 113, and ultimately to the brain.
Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104. To improve impaired hearing, auditory prostheses have been developed. For example, when the impairment is related to operation of the middle ear 103, a conventional hearing aid may be used to provide acoustic-mechanical stimulation to the auditory system in the form of amplified sound. Or when the impairment is associated with the cochlea 104, a cochlear implant with an implanted electrode contact can electrically stimulate auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along the electrode.
FIG. 1 also shows some components of a typical cochlear implant system which includes an external microphone that provides an audio signal input to an external signal processor 111 where various signal processing schemes can be implemented. The processed signal is then converted into a digital data format, such as a sequence of data frames, for transmission into the implant 108 by external transmitter coil 107. Besides receiving the processed audio information, the implant 108 also performs additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through an electrode lead 109 to an implanted electrode array 110. Typically, this electrode array 110 includes multiple electrodes on its surface that provide selective stimulation of the cochlea 104.
To allow optimal performance, an ear implant system such as a cochlear implant needs to be adapted for the individual user. This adaptation process is normally referred to as the fitting process, or simply fitting. During the fitting process, several parameters within the cochlear implant system are set to best match the needs of the individual user. The result of this fitting process is normally referred to as a fitting or a map. One example of such a parameter is the Maximum Comfortable Loudness (MCL) level, i.e. the charge or stimulation level which produces a hearing impression which is comfortably loud but just without being too loud. Another example is Threshold (THR) level, which is the charge or stimulation level which produces a just audible, or alternatively a just-inaudible, hearing sensation. Other possible parameters concern, for example, the frequency distribution across the channels of the cochlear implant, or the loudness growth curve (commonly referred to as the maplaw). Among these parameters, the MCL is the one which invariably needs to be fitted to the each cochlear implant user individually, whereas for the other parameters reliable default values exist in general
Several methods are used to obtain the necessary fitting parameters. For example, psychoacoustic methods as well as objective measures are used to obtain MCL and THR levels. In psychoacoustic methods, the cochlear implant user is asked to indicate which stimulation level produces a hearing sensation of the desired loudness (e.g. maximum comfortably loud for MCL or just not audible for THR). In objective measures, it is attempted to derive these levels from evoked potential or objective response measurements. One possible objective response used in this context is the threshold at which the stapedial reflex occurs, also called the electrical stapedial reflex threshold (ESRT). The ESRT is well correlated with MCL levels and can thus be used to determine MCL levels. Other objective measures concern evoked potentials measured along the cochlear pathway, from peripheral responses measured inside the cochlea to responses from the brainstem to responses from the auditory cortex, for example, measurements of electrically evoked compound action potential (ECAP) and/or electrically evoked auditory brain stem response (EABR).
Whatever method is used, the fitting of a cochlear implant is a relatively time-consuming procedure. Ideally, all channels in an implant need to be assessed individually to obtain the necessary parameters, which can require attention and cooperation by the implant user for a relatively long period of time. Thus, in children and other individuals with a restricted attention span or lesser ability to comply with the measurement paradigm, this procedure presents a serious challenge to both the clinician and the patient. Therefore, procedures allowing accelerated fitting are desirable.
Several methods are known for accelerating the fitting process in cochlear implants. One method concerns the use of interpolation. Necessary channel-specific parameters are obtained using psychoacoustics or objective measures for only a subset of the available channels in a cochlear implant, and the parameters on the remaining channels are estimated using those obtained parameters. For example, MCL levels may be obtained just for odd-number channels in the cochlear implant (i.e. channels 1, 3, 5 etc.) using any of the known methods, and MCL levels for the even-number channels are determined by interpolation. In many cases, this may be a linear interpolation where parameters for unmeasured channels in between measured channels are derived by linear interpolation. However, any other method of interpolation, or in more general terms, any other method of deriving estimated parameter values from measured parameters values can be used as well. In such endeavors, the question remains on which channels should parameters be obtained using psychoacoustics or objective measures, and on which channels should parameters be estimated based on the previously obtained parameters. To date, no method exists to guide the clinician in selecting the most appropriate channels for measuring or estimating these fitting parameters.